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United States Patent |
6,023,110
|
Henrion
,   et al.
|
February 8, 2000
|
Switching equipment
Abstract
Switching equipment with a contactor (CE) and a circuit breaker (BR)
located ahead of the contactor. For detection of welded-together contacts
of the contactor, this is provided with means (SC) adapted, a certain time
after an opening order to the contactor, to apply a voltage pulse to the
operating coil (12) of the contactor and to compare the current response
of the operating coil with a comparison level for forming a detection
signal (s.sub.d), which is supplied to the circuit breaker. Upon detection
of welded-together contacts, the detection signal triggers an opening of
the circuit breaker for disconnection of the contactor.
Inventors:
|
Henrion; Claude (Saint Priest, FR);
Johansson; Gunnar (Vaster.ang.s, SE);
Stephansson; Paul (Enebyberg, SE);
Vefling; Harald (Eiksmarka, NO)
|
Assignee:
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ABB Research Ltd. (Vasteras, SE)
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Appl. No.:
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952933 |
Filed:
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March 2, 1998 |
PCT Filed:
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June 12, 1996
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PCT NO:
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PCT/SE96/00762
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371 Date:
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March 2, 1998
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102(e) Date:
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March 2, 1998
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PCT PUB.NO.:
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WO96/42098 |
PCT PUB. Date:
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December 27, 1996 |
Foreign Application Priority Data
Current U.S. Class: |
307/125; 324/422; 340/644 |
Intern'l Class: |
H01H 073/12 |
Field of Search: |
307/112,116,125,130,131
324/415-424
340/638,644
|
References Cited
U.S. Patent Documents
5204633 | Apr., 1993 | Ahladas et al. | 324/654.
|
5243291 | Sep., 1993 | Umemura | 324/418.
|
5455733 | Oct., 1995 | Waggamon | 361/115.
|
5774323 | Jun., 1998 | Innes et al. | 361/187.
|
Primary Examiner: Paladini; Albert W.
Attorney, Agent or Firm: Pollock, Vande Sande & Amernick
Claims
We claim:
1. Switching equipment with an electromagnetic contactor and a circuit
breaker located ahead of the contactor, the contactor having an operating
magnetic circuit with a magnetic core, an operating coil, an armature
which moves in dependence on the current through the operating coil, and a
number of contacts which are influenced by the armature, the switching
equipment further comprising detection circuits for sensing the reluctance
of the operating magnetic circuit and, in dependence on the measured
reluctance, generating a signal which indicates an incomplete opening of
the contactor caused by welded-together contacts, said signal being
supplied to the circuit breaker so that, upon detection of welded-together
contacts, the contactor is disconnected by opening of the circuit breaker,
wherein the detection circuits measure the reluctance of the operating
magnetic circuit by sensing the inductance of an inductance measuring coil
surrounding the magnetic core, and wherein the detection circuits apply to
the inductance measuring coil a voltage pulse and detect the inductance of
the coil on the basis of the current response of the coil.
2. Switching equipment according to claim 1, wherein the inductance
measuring coil consists of the operating coil.
3. Switching equipment according to claim 1, wherein the detection circuits
sense the reluctance of the operating magnetic circuit when a time
interval has elapsed after an opening order received by the contactor.
4. Switching equipment according to claim 1, wherein the detection
circuits, at a predetermined time after the start of the voltage pulse,
compare the current response with a reference level.
5. Switching equipment according to claim 4, wherein the detection circuits
apply said voltage pulse to the inductance measuring coil by connecting
the coil to a voltage source and the detection circuits comprise means for
forming the reference level in dependence on the voltage of the voltage
source.
6. Switching equipment according to claim 1, wherein the detection
circuits, when reaching a predetermined current level, compare the time
interval elapsed since the start of the voltage pulse with a predetermined
time interval.
7. Switching equipment according to claim 1, wherein the operating coil is
connected to a voltage source in series with a switching member for
controlling the current through the coil, and the detection circuits
further comprise means adapted for controlling the switching means into a
conducting state for applying said voltage pulse across the operating
coil.
Description
TECHNICAL FIELD
The invention relates to switching equipment with an electromagnetic
contactor and a circuit breaker which is located ahead of the contactor.
The contactor has an operating magnetic circuit with a magnetic core, an
operating coil and an armature which moves in dependence on the current
flow through the operating coil. Furthermore, the contactor has a number
of contacts which are influenced by the armature.
BACKGROUND OF THE INVENTION
Electromagnetic contactors are known and have been used for a long time,
for example as switching means between a voltage source and an electric
motor. One problem with such contactors is that one or a few of the
contact pairs of a contactor may become fixed to each other by welding,
and the risk of this is greater at high currents. Such welding together of
contact pairs may, for example, be caused by contact bouncing when closing
the contactor towards a high making current of an electric motor.
The fact that one or more contact pairs become fixed by welding may entail
serious harmful effects. Upon an opening signal to a contactor with a
welded-together contact pair, the armature will move a certain distance in
the opening direction, because of the resilience in the mechanical
coupling, and then stop in an intermediate position. This may cause arcs
in the contact pairs which are not welded together, and fire, explosion or
other damage to the contactor and other equipment. In many applications,
it may also, and independently thereof, cause serious consequences that a
contactor does not open when, according to a supplied opening signal, it
should have opened.
SUMMARY OF THE INVENTION
The object of the invention is to provide switching equipment of the kind
mentioned in the introductory part of the description, in which the risk
of damage and other inconvenience, which may otherwise arise during an
incomplete opening of the contactor caused by welded-together contacts, is
eliminated in a simple manner.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be explained in greater detail in the following with
reference to the accompanying FIGS. 1-4 wherin.
FIG. 1 shows switching equipment according to the invention, connected in
the supply conduit of an ac motor.
FIG. 2 shows the composition of the control equipment of the contactor.
FIG. 3 shows the control circuit included in the control equipment.
FIG. 4 shows how some of the quantities occurring in the switching
equipment vary with time during an opening operation.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows switching equipment according to the invention connected to
the line between a three-phase motor M and an alternating-voltage power
supply network N. The switching equipment comprises contactor equipment CE
and a circuit breaker BR located ahead of the contact equipment (by "ahead
of" is meant that the circuit breaker is arranged between the contactor
equipment and the supply network.) The function of the switching equipment
is to connect, in dependence on a control signal s.sub.c, the motor to or
disconnect the motor from the supply voltage. The control signal may be
obtained in a known manner from superordinate control equipment or be
supplied manually. The contactor equipment is usually adapted to also to
serve as thermal overload protection means for the motor and then receives
an opening signal from a current-sensing protective circuit (not shown).
The circuit breaker BR, which in a known way is adapted to trip at
overcurrents, serves as overcurrent protection device. As shown in the
figure, the circuit breaker also receives a tripping signal s.sub.d from
the contactor equipment for opening of the circuit breaker if contacts of
the contactor have become fixed by welding.
In the usual manner, the contactor equipment has a bank of contacts 10
which, in the three-phase application shown, has three contacts, one for
each phase. Via a resilient mechanical link 14, the contacts are
mechanically connected to the armature 13 of the operating magnet 11 of
the contactor, which magnet has an operating coil 12. The contactor
equipment has control equipment SC which receives the control signal
s.sub.c. Upon signals for closing, the control equipment feeds a current I
to the operating coil and maintains this current at a desired value.
Further, the control circuit comprises circuits for detecting contacts
which have become fixed by welding and for supplying a detection signal
s.sub.d for tripping the circuit breaker BR if it is detected that
contacts have become fixed by welding.
FIG. 2 shows the composition of the control equipment SC. The operating
coil 12 is connected, in series with a resistor R1, a switching transistor
TR1 and a measuring resistor R.sub.m, to a supply voltage source with a
direct voltage +U. A bypass diode D is connected in parallel with the
operating coil. A measuring voltage u.sub.m, corresponding to the current
I through the coil (in case of a non-conducting diode D), is obtained
across the measuring resistor. The transistor TR1 is used, in the manner
which will be described below, to control the current through the coil 12
upon closing of the contactor and in the closed position, as well as for
applying a voltage pulse to the coil for detection of contacts being fixed
by welding. An RC circuit comprising a resistor R.sub.C and a capacitor C
is connected to the supply voltage source. The capacitor may be connected
to the measuring resistor with the aid of a switching transistor TR2. A
control circuit CC receives the control signal s.sub.c and the measurement
signals u.sub.m and u.sub.c, the latter corresponding to the capacitor
voltage, and delivers control signals s.sub.I and s.sub.rs to the
transistors TR1 and TR2 and the tripping signal s.sub.d to the circuit
breaker BR.
FIG. 3 shows the composition of the control circuit CC. The measurement
signal u.sub.m is supplied to an input of a level-sensing circuit NV1, and
to the second, inverting input there is supplied a reference value I.sub.0
which corresponds to the desired current through the operating coil 12
when the contactor is closed. The circuit NV1 has a certain hysteresis and
delivers an output signal which becomes "0" if the coil current rises
above an upper limit value and which becomes "1" if the current drops
below a lower limit. The output signal of the circuit is forwarded via an
AND circuit OG1 to an OR circuit EG, the output signal s.sub.I of which
controls the transistor TR1, which is on at s.sub.I =1 and off if s.sub.I
=0. The AND circuit releases the signal from NV1 and hence the control
signals to the transistor if there is an order for a closed contactor,
that is, if the control signal s.sub.c is "1". The circuit described so
far thus controls, in a known manner, by pulsing the transistor TR1, the
current through the operating coil to the desired value independently of
supply voltages varying within wide limits. Circuits of this kind for
control of the current through the operating coil of a contactor are
known, for example from the published patent applications EP 0 136 968 A3
and WO 86/01332.
The control signal s.sub.c is also supplied to a monostable circuit MV1
which is triggered when the control signal changes from "1" to "0", that
is, when an opening signal is supplied to the contactor. The circuit MV1
then delivers a pulse with a duration t.sub.1 so adjusted that the
contactor has normally had time to assume the open position at the end of
the pulse. The output signal from the circuit MV1 is supplied to two
additional monostable circuits MV2 and MV3, which are both triggered at
the end of the pulse from MV1, that is, the time t.sub.1 after an opening
order to the contactor. The circuit MV2 delivers a short control pulse
s.sub.rs to the transistor TR2, which thereby becomes conducting for a
short moment and causes the capacitor voltage u.sub.c to become identical
with the voltage u.sub.m across the measuring resistor. The circuit MV3
delivers a pulse with the duration t.sub.2 which corresponds to the length
of the detection interval and which, for example, may be 0.1 ms. This
pulse is supplied to the transistor TR1 via the OR circuit EG and controls
the transistor to a conducting state for the duration of the pulse. In
this way, the supply voltage U is continuously applied to the operating
coil 12 for the duration of the detection pulse. The pulse from the
circuit MV3 is also supplied to a fourth monostable circuit MV4, which is
triggered at the end of the pulse from MV3, that is, at the end of the
detection interval, and then delivers a short signal to a second AND
circuit OG2.
A level-sensing circuit NV2 is supplied with the signals u.sub.c and
u.sub.m, the latter with reversed sign. If u.sub.c >u.sub.m, the output
signal of the circuit is "1", and when, at the end of the detection
interval, the circuit OG2 receives a pulse from the circuit MV4, a signal
s.sub.d is delivered which indicates whether any of the contacts of the
contactor has been fixed by welding. This signal is supplied to the
circuit breaker BR and triggers an immediate opening of the circuit
breaker.
FIG. 4 illustrates the process of some of the quantities occurring in the
switching equipment. At the top in the figure, the control signal s.sub.c
is shown, which is "1" up to t=t.sub.0, that is, for t.ltoreq.t.sub.0 the
contactor is in the closed position. The control equipment controls the
current I through the operating coil by pulsing the transistor TR1, the
control signal s.sub.I of which is shown below the control signal s.sub.c
in the figure. Below this, the current I is shown and as is clear from the
diagram this is controlled so that its mean value corresponds to the
reference value I.sub.0.
At t=t.sub.0 an opening order is given, and the control signal s.sub.c
becomes "0". The coil current I then decreases exponentially towards zero.
After the time t.sub.1 determined by the circuit MV1, the detection
interval is started. A short control pulse s.sub.rs is supplied to the
transistor TR2, which becomes conducting and causes the capacitor voltage
u.sub.c to become identical with the measuring voltage u.sub.m. At the
same time, the transistor TR1 is controlled to the conducting state and
the supply voltage U is applied to the operating coil. Its current I then
increases at a rate which is dependent on the magnitude of the supply
voltage and on the inductance of the operating coil (the coil resistance
is assumed to be constant). The inductance, in its turn, is dependent on
the reluctance (the magnetic resistance) of the magnetic circuit of the
operating magnet. The reluctance varies, in turn, with the air gap between
the armature and the magnetic core. It is smallest in a fully closed
position, when the air gap is zero, and greatest in a fully opened
position when the air gap has its greatest value. If one or more of the
contacts of the contactor should be fixed by welding upon an opening
operation, the armature, because of the resilient mechanical coupling
between the armature and the contacts, will move a certain distance until
the welded contact or contacts prevent continued movement. The armature
then stops in an intermediate position, where the reluctance assumes a
value between its greatest and its smallest value.
The two lowermost diagrams in FIG. 4 show how the current I and the
measurement signal u.sub.m vary during the detection interval. The normal
process is shown in dotted lines. The air gap has had time to assume its
greatest value even at the beginning of the detection interval, the
reluctance is great and the coil inductance small, and therefore the coil
current increases rapidly. The unbroken lines show the process if at least
one contact is fixed by welding. The reluctance then becomes lower and the
coil inductance greater, and the current increases more slowly. The time
constant of the RC circuit RC-C is so chosen that the signal u.sub.c
increases more slowly than the coil current in the normal case but faster
than the coil current in case of a contact which is fixed by welding. At
the end of the detection interval, therefore, in the normal case u.sub.m
>u.sub.c and no output signal is obtained from the circuit NV2. In the
case of a welded contact, on the other hand, at the end of the interval
u.sub.m <u.sub.c, the output signal from the circuit NV2 becomes "1" and a
tripping signal s.sub.d is delivered to the circuit breaker BR. This
causes the circuit breaker to immediately trip and prevent further damage
to the contactor and damage to the other equipment.
By supplying the RC circuit in the above-described embodiment from the same
supply voltage source as the operating coil, the important advantage is
obtained that variations in the supply voltage will influence the rate of
growth of the comparison quantity u.sub.c in the same way and to the same
extent as the variations influence the rate of growth of the coil current.
The detection of contacts fixed by welding therefore becomes correct even
if the supply voltage varies, and switching equipment according to the
invention may be connected to different supply voltages without
influencing the detection.
By setting the comparison quantity u.sub.c, at the beginning of the
detection interval, always equal to the value which corresponds to the
coil current, the detection becomes correct independently of the magnitude
of the coil current at the beginning of the interval. This is an important
advantage and makes it possible, for example, without negatively
influencing the accuracy of the detection, to initiate the detection, and
when necessary achieve disconnection of the contactor, earlier than what
would otherwise have been possible, thus reducing the harmful effects of
contacts fixed by welding.
From experience, in a typical contactor, the reluctance in the open
position is about 3-10 times greater than in the closed position, that is,
the coil inductance is about 3-10 times lower. This relatively large ratio
makes possible a reliable detection of contacts fixed by welding by
utilizing a reluctance determination. Further, the method described above
is simple and economically advantageous. It requires no transducers or
extra connections of the contactor and only a relatively simple
supplementation of the static parts of the contactor equipment. In the
case described above, where the invention is applied to contactor
equipment which is provided with means for control of the current of the
operating coil, the already existing control means are utilized, and the
only thing that is required is a moderate supplementation of the
signal-processing circuits of the equipment.
The equipment described above is only an example, and switching equipment
according to the invention can be designed in a plurality of other ways
than that described above.
According to the invention, the change in the reluctance of the operating
magnet, in dependence on the position of the armature, is utilized for the
detection. Quantities equivalent to the reluctance may, of course,
alternatively be used within the scope of the invention, for example the
inverted value of the reluctance, the permeance, or the coil inductance
proportional to the permeance.
In the above description, the operating coil and its current-controlling
means have been used for the reluctance determination, which is a simple
and advantageous embodiment, but alternatively there may be used, for
example, a separate inductance measuring coil.
In the embodiment described above, a measure of the reluctance is formed by
determining the current change during a time interval of a predetermined
length. Alternatively, of course, a measure of the reluctance may be
formed by determining the time for a predetermined current change.
The resetting of the comparison quantity (by closing the transistor TR2)
described above causes the measurement to be completely independent of
which value the current coil has at the beginning of the detection
interval.
The invention has been described above with reference to a contactor, the
contacts of which are open when the contactor is in the open position and
closed in the closed position. The invention can also be applied to a
contactor with at least some contact which is closed in the open position
of the contactor and where thus the contactor, when this contact has been
fixed by welding, may stop in an intermediate position when closing the
contactor.
In the embodiment described above, the control and detection equipment is a
mixture of analog and digital circuits, but the corresponding functions
may be obtained in other ways, for example with the aid of an
appropriately programmed microprocessor.
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